Transmission oil coolers for automotive vehicles are often installed in the vehicle radiator so that the engine coolant flows over the oil cooler and heat is transferred from the transmission oil to the engine coolant. The oil cooler should be characterized by compactness to fit within the tank of a radiator, low resistance to oil flow, strength to contain the pressure of the transmission oil, and high efficiency of heat transfer. The heat transfer efficiency and size are related since a smaller unit may be used for a given thermal transfer requirement if the efficiency is increased.
Heretofore, oil coolers have used a plate type heat exchanger comprising at least one pair of spaced plates secured together at their margins to define a passageway which contains the oil flow and has a conductive insert or center to enhance the heat transfer. Such a prior art device is shown in FIG. 1 and includes a female strip 10 with upstanding side margins 12 and a cooperating male strip 14 secured to the side margins 12 to form an elongated enclosure. A port 16 at each end of the assembly allow oil flow in one port, through the enclosure and out of the other port. A center 18, best shown in FIG. 2, comprises a stamped metal foil formed into staggered step-like undulations. The stamped foil is brazed to the plates or strips 10 and 14. The passageway between the plates is then configured by the center 18 into a plurality of meandering flow paths having a combined resultant flow in the direction shown by the arrow 19. The center 18 causes turbulence which enhances heat transfer and conducts heat from the oil to the plates 10, 14, thereby improving efficiency over a plain plate pair without a center 18.
Other types of heat exchangers have used other kinds of centers. Scarselleta U.S. Pat. No. 4,693,307 shows a center design used in tube and fin heat exchangers suitable for automotive radiators. In that patent a radiator design is disclosed wherein a corrugated sheet formed into a plurality of side-by side fins is sandwiched between flat tubes carrying engine coolant, with the fins being used to dissipate heat from the tubes to the air which flows through the fins. A conventional multi-louver fin is shown in FIG. 11 of Scarselletta which has louvers struck out of the plane of each fin and the louvers cover most of the fin area. Other fin designs shown in that patent are the hybrid fin variety which alternates plain fin surface with louvered areas.
The heat exchanger designs for radiators, do not directly apply to oil coolers because of differing constraints on size, corrosion resistance, pressure and thermal capacity. Thus different materials are used and the fin heights are different. To obtain the pressure capability and corrosion resistance for oil coolers, steel centers are used rather than aluminum which is commonly used for radiators and the center height is much less for the steel due to the lower heat conductivity of steel. Thus a number of design considerations require independent designs for radiators and oil coolers.